This invention relates to modems.
A modem is a device for transmitting and receiving digital information over a bandwidth-limited channel, such as a telephone line. Typically, two modems are required to communicate over the channel in a given direction, one modem located at one end of the channel to transmit a signal and another modem at the other end of the channel to receive the signal. The transmitting modem employs an appropriate modulation technique to convert the digital signal into a form which may be transmitted over the channel and the receiving modem demodulates the received signal to recover the digital information.
Data transmission can occur in both directions over a channel and it can be in either half-duplex mode or fullduplex mode. In half-duplex mode, communication occurs only in one of the directions at any given time. In full-duplex mode, communication occurs in both directions at once. In both modes, the rate at which data can be transmitted depends critically on the quality of the channel, that is, on the amount and character of noise which the channel adds to the transmitted signal. Typical channel noise includes idle noise, which is present even when no signal is being transmitted, and signal induced distortion noise impairs the quality of the channel and reduces the signal-to-noise ratio of the transmitted signals. Thus, a higher noise level on the channel, or equivalently, lower channel quality, generally means that a lower communication rate must be used in order to achieve a reliable, error-free transfer of data.
In full-duplex mode, there is an additional impairment to channel quality besides channel noise. The additional impairment is referred to as echo. Some of the transmitted signal inevitably feeds directly back to the receiver side of the sending modem where it appears as an undesired near echo signal that interferes with the received signal sent by a remote modem. In addition, because impedance mismatches in the network are unavoidable, some of the sending modem's transmitted signal is reflected back from the distant modem and from any other point at which there is an impedance mismatch to the sending modem where it appears as a far echo signal which further corrupts the signal transmitted from the distant modem. The combination of the near and far echo are referred to as the combined echo.
Methods have been developed for separately dealing with the two types of channel impairment, namely, the channel distortion and the combined echo. For example, modems are available which have the ability to select the communication rate for half-duplex communication based upon the quality of the channel. Some of these modems determine the quality of the channel by measuring the amount of noise which is present in the received signal. Then, based upon a negotiation with the other modem with which they are communicating, these modems select from among several available communication rates the particular rate which will yield the desired signal-to-noise ratio, i.e., the desired performance given the noise present on the channel.
Modems are available which include an echo canceler to improve full-duplex communication. In principle, the echo canceler operates by generating replicas of the near and far echo signals which are then subtracted from the received signal to yield a relatively echo-free signal.
Modems which incorporate the above-described methods of dealing with channel impairments generally conform to specific communication protocols to assure that they can communicate with other modems which are made by other manufacturers and which may not incorporate the same methods of dealing with channel impairments. One important organization which develops such protocols is the CCITT group within the International Telecommunication Union (ITU). The CCITT has promulgated or recommended various protocols for half-duplex and full-duplex communication. For example, a current recommendation for the family of 2-wire, duplex modems is the V.32 specification. The V.32 specification defines the signaling sequences and timing of the sequences which two modems must use in order to establish and continue communications with each other. Since such standards are widely adopted by modem manufacturers, it is important that any features or capabilities which are developed for newer modems conform to such specifications.